Limit markaby version to < 0.9.1 in CI on Ruby <2.7

markaby 0.9.1 was released in early December with syntax that
required version Ruby 2.7, but required_ruby_version was not
set.  This was fixed in markaby 0.9.2, but because 0.9.1 still
exists, bundler will choose that on older Ruby versions unless
you restrict the version manually.

.ci.gemfile

@@ -10,7 +10,6 @@ gem 'erubi'
 gem 'erubis'
 gem 'liquid'
 gem 'livescript'
-gem 'markaby'
 gem 'nokogiri'
 gem 'pdf-reader'
 gem 'prawn', '>= 2.0.0'
@@ -30,6 +29,12 @@ if RUBY_VERSION < '2.3'
   gem 'execjs', '< 2.9'
 end
 
+if RUBY_VERSION < '2.7'
+  gem 'markaby', '< 0.9.1'
+else
+  gem 'markaby'
+end
+
 if RUBY_VERSION < '2.1'
   gem 'kramdown', '< 1.1'
 else